The pollen, or sperm, from a red maple tree is unable to fertilize the eggs from a sugar maple tree. this is an example of what type of isolating mechanism?

Gametic isolation.

Explanation:

To prevent the mating between the individuals that belong to different species there are two types of barriers that exist in nature - pre-zygotic and post-zygotic. The prezygotic barriers are the ones which prevents the mating to occur and gametic isolation is one of the types of such kind of a barrier. In gametic isolation, the gametes from different species are unable to fertilize each other even if they meet. The gametic isolation is possible because of the incompatibility of the gametes belonging to different species. Therefore, in the given question, since red maple and sugar maple are different species, their sperm and egg are unable to form a zygote and thus, this is an example of gametic isolation.

Gamete isolation

which of the following lists the level of taxonomic classification in order from the most inclusive to the least inclusive

domain - kingdon - phylum - class - order